[USRP-users] (Mightbe) an impedance mismatching problem

Matt Ettus matt at ettus.com
Wed Jul 29 19:12:29 EDT 2015


You didn't say which daughterboard you are using, but I assume it is LFRX.
That has a 50 ohm input impedance, and your device might not be capable of
driving it.  You also need to make sure everything shares the same ground.

Matt

On Tue, Jul 28, 2015 at 5:48 AM, Jeon via USRP-users <
usrp-users at lists.ettus.com> wrote:

> I am building a photo-detecting circuit, whose output will be fed into a
> USRP N210. (Also have USRP1, E100, B200... But, currently I am using N210.)
>
> Here's a circuit that I built:
>
>
>
> To connect with the USRP (drawn as switch in the circuit. It's just a
> logical representation whether circuit output is fed into the USRP or not)
> I use BNC- Alligator CLIP SP RG58 coaxial cable of 1 meter. Since the USRP
> has the SMA port, I also put the BNC JACK to SMA PLUG adapter.
> A distributor says that the cable has an impedance of 50 ohm. The pages
> below are in Korean. Just in case.
>
> - BNC coaxial: http://eleparts.co.kr/EPX33HV8
> - BNCJ-SMAP adapter: http://eleparts.co.kr/EPX33PCF
>
> Back to the circuit, I've tested the circuit with another LED driving
> circuit. Without connecting the USRP.
>
>
>
> Blue is a state of an LED and yellow is a brightness reading (output of
> the op amp).
> I think it's not that pretty, but it's not that bad... It's OK...
>
> And then after, I connect the USRP at the output of the op amp. And did
> the same test. And the result is:
>
>
>
> First, it can be easily found that the blue reading and the yellow reading
> are not having a same HIGH/LOW pattern.
> In addition, a vertical scale is reduced to nearly 20 %. (The first
> oscilloscope has a scale of 500 mV, and the second one ahs a scale of 100
> mV.)
>
> With a little knowledge about circuit and hardware, I guess this problem
> is mainly caused by impedance mismatch.
>
> In this situation, I am curious whether this phenomenon is an impedance
> mismatch or not.
> If it is, how can I solve the problem with an arbitrary LED driving
> frequency?
> (Currently, the LED is driven with a frequency of 500 Hz, but I am
> planning to increase the frequency up to 800 kHz and more.)
>
> If it has nothing to do with impedance mismatch, what causes this
> phenomenon and how could I solve it?
>
> I apologize in advance if you feel this question does not suit this USRP
> users mailing list, but does suit other places such as EE stackexchange.
>
> Regards,
> Jeon.
>
>
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>
>
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